摘要:

AbstractThe Madden‐Julian Oscillation (MJO) is examined using 20‐years of outgoing long‐wave radiation and National Centers for Environmental Prediction/National Center for Atmospheric Research re‐analysis data. Two mechanisms for the eastward propagation and regeneration of the convective anomalies are suggested.The first is a local mechanism operating over the warm‐pool region. At the phase of the MJO with a dipole structure to the convection anomalies, there is enhanced tropical convection over the eastern Indian Ocean and reduced convection over the western Pacific. Over the equatorial western Indian Ocean, the equatorial Rossby wave response to the west of the enhanced convection includes a region of anomalous surface divergence associated with the anomalous surface westerlies and pressure ridge. This lends to suppress ascent in the boundary layer and shuts off the deep convection, eventually leading to a convective anomaly of the opposite sign. Over the Indonesian sector, the equatorial Kelvin wave response to the east of the enhanced convection includes a region of anomalous surface convergence into the anomalous equatorial surface easterlies and pressure trough, which will tend to favour convection in this region. The Indonesian sector is also influenced by an equatorial Rossby wave response (of opposite sign) to the west of diereducedconvection over the western Pacific, which also has a region of anomalous surface convergence associated with its anomalous equatorial surface easterlies and pressure trough. Hence, convective anomalies of either sign tend to erode themselves from the west and initiate a convective anomaly of opposite sign via their equatorial Rossby wave response, and expand to the east via their equatorial Kelvin wave response.The second mechanism is global, involving an anomaly completing a circuit of the equator. Enhanced convection over the tropical western Pacific excites a negative mean‐sea‐level pressure (m.s.l.p.) anomaly which radiates rapidly eastward as a dry equatorial Kelvin wave at approximately 35 m s−1over the eastern Pacific. It is blocked by the orographic barrier of the Andes and Central America for several days before propagating through the gap at Panama. After rapidly propagating as a dry equatorial Kelvin wave over the Atlantic, the m.s.l.p. anomaly is delayed further by the East African Highlands before it reaches the Indian Ocean and coincides with the development of enhanced convection at the start of the

ISSN:0035-9009    

DOI:10.1002/qj.49712656902

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractA simple two‐layer analogue model is used to elucidate aspects of vortex motion in a vertically sheared zonal flow. The model is based on the idea that a vortex can be considered as the sum of a pair of barotropic vortices, one whose vorticity, or potential vorticity, resides in the upper layer and the other whose vorticity resides in the lower layer. Each vortex has an associated tangential velocity distribution in the other layer, which advects the vortex in that layer. The strength of this velocity distribution is characterized by a coupling parameter, which, in the case of quasi‐geostrophic vortices, is related to the Rossby depth scale. Besides their mutual advection, the component vortices are differentially advected by vertical shear. The model leads to a set of coupled ordinary differential equations for the motion of each component vortex, which may be solved analytically in certain circumstances. The calculations indicate two types of flow behaviour according to the strength of the component vortices, the degree of vertical coupling and the strength of the shear. For weak shear and/or strong vortices and strong coupling, the vortices rotate around each other as their mean centre translates with a fraction of the mean zonal flow. For Strong shear and/or weak vortices and weak coupling, the vortices undergo a partial rotation while they are in proximity, but become progressively separated by the shear. The calculations are an aid to understanding the range of behaviour of vortices in shear in numerical calculations by other authors, and it is reasonable to presume that the processes represented by the model are fundamental processes in tropical cyclones also.The analogue model is evaluated in the context of quasi‐geostrophic theory, where the breakdown into the component vortices can be accomplished and where the complete problem can be solved numerically without approximation. The results of the quasi‐geostrophic model are contrasted with those of other recent studies of baroclinic vortices in the absence of vertica

ISSN:0035-9009    

DOI:10.1002/qj.49712656903

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractSolutions are obtained for linear hydrostatic disturbances propagating parallel to the face of an uninterrupted topographic step in an infinitely deep, stably stratified fluid on an β‐plane. These waves are vertically trapped because their frequencies are smaller than the Coriolis parameter and the height of the topographic step is finite. These waves are referred to asstep‐trapped Kelvin waves, because they are dynamically similar to internal Kelvin waves throughout the layer of fluid below the top of the topographic step. These waves appear to provide an idealized, semi‐analytic model for the coastally trapped disturbances observed to propagate parallel to mountainous coastlines in several parts of the world.Computations are performed for a basic state with uniform static stability and for a three‐layer basic state in which the two lowest layers represent the marine boundary layer and a strong capping inversion. One might suppose that the linear dynamics of hydrostatic disturbances in the three‐layer basic state could be well approximated by a reduced‐gravity shallow‐water model, but this is not the case. In particular, the reduced‐gravity shallow‐water model does not provide reliable estimates for the phase speed of linear step‐trapped Kelvin waves. This defect suggests that detailed quantitative comparisons between marine boundary‐layer flows and the reduced‐gravity shallow‐water system may not have any intrinsic physical significance. Nevertheless, these results do not preclude the possibility of constructing useful qualitative analogies between marine boundary‐layer flows and the reduced

ISSN:0035-9009    

DOI:10.1002/qj.49712656904

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractThe interaction between two storm tracks is analysed by using idealized experiments with a simplified global‐circulation model. A zonal distance of 180° between the storm tracks drives a retrograde travelling zonal wave‐number‐two wave with an amplitude modulation of about 50 days. The stream‐function tendency equation demonstrates that the amplitude modulation is excited in the storm‐track regions by the high‐frequency (periods between two and eight days) eddy forcing; ‘spatial resonance’ of a retrograde propagating zonal wave‐number‐two Rossby wave and high‐frequency eddy forcing are proposed as the amplifying mechanisms, while friction and self interaction between the low‐frequency eddies ensure damping. Sensitivity experiments indicate that the pronounced modulation of 50 days vanishes with decreasing zonal distance between the storm tracks, thereby enhancing the intensity and dur

ISSN:0035-9009    

DOI:10.1002/qj.49712656905

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractA general, linear vertical structure equation for the vertical velocity component, including explicit forcing terms in the momentum, thermodynamic and continuity equations, is derived for horizontally‐homogeneous flows. The basic flow is assumed to depend on height alone and is in geostrophic and hydrostatic balance. Scale analysis is used to show that this equation incorporates a variety of familiar special cases including the lee‐wave equation, Eady's equation and the quasi‐geostrophic omega equation, the different flow regimes being identified in terms of the Rossby, Froude and Richardson numbers. Using the vertical structure equation, a wave‐stress conservation principle is derived that is valid for basic flows whose magnitude and direction vary with height. In addition to providing some unification to the many flavours of vertical velocity equation in the literature, this derivation was motivated by the need to provide a starting point for a wide class of analytical problems in the study of baroclinic instability and inertia‐gravity wave

ISSN:0035-9009    

DOI:10.1002/qj.49712656906

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractThe nonlinear behaviour of quasi‐stationary planetary waves that are excited by mid‐latitude orographic forcing is considered in a global shallow‐water model taken to represent the upper troposphere. The waves propagate toward low latitudes where the background flow is normally weak and the waves are therefore likely to break. Nonlinear pseudomomentum conservation relations are used to quantify the absorption‐reflection behaviour of the wave‐breaking region. Two different flow scenarios are represented: (i) initial states without a representation of the Hadley circulation, but where the axisymmetric equatorial background flow changes from being weak and easterly to moderate westerly; (ii) initial states that include a representation of the Hadley circulation and that have weak equatorial easterlies.Based on linear arguments, both (i) and (ii) are expected to influence the progression of the wave train. The nonlinear behaviour in the presence of low‐latitude westerly background flows is different from linear predictions. For large‐amplitude forcing, wave breaking takes place even though there is no zero‐wind line in the initial slate, and the cross‐equatorial wave propagation that look place for small‐amplitude forcing is stopped before it can reach the equator. Nonlinear reflection is found to take place back into the hemisphere of origin but not across to the other hemisphere.In the presence of a Hadley circulation representative of winter conditions, the nonlinear reflection takes longer to get established, i.e. it requires more forcing, but a reflected wave train is still present in the numerical simulations, both for a longitudinally symmetric forcing and for the more realistic case of an isolated forcing. A summer Hadley circulation allows wave activity to get to the winter hemisphere. As the forcing is increased, wave breaking occurs and eventually n

ISSN:0035-9009    

DOI:10.1002/qj.49712656907

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractIt has been noted previously that during frontal collapse dynamical processes lead to the formation of potential vorticity (PV) anomalies in the vicinity of the surface front. These processes can either be associated with diffusion in the presence of the tight temperature gradients or with intrusion into the atmosphere of the vanishingly thin layer adjacent to the surface. This paper explores the dynamical consequences of these PV anomalies on the parent baroclinic wave cyclone. The method used is to find the vertical motion attributable to these anomalies. The location and magnitude of this vertical motion is clearly key to the dynamical influence of the anomalies. As we are dealing with a three‐dimensional evolving cyclone with an emergent tropopause fold, the calculation of vertical motion needs to be capable of accounting for the role of the highly deformed tropopause. Hence we develop and use a nonlinear balance model approach. The result shows that the PV anomalies near the surface front induce downward vertical motion at the tropopause fold, and thus they act to amplify this feature. This role of diffusion associated with surface frontal collapse is the key finding of this paper. An analysis is made of the role of upper‐ and lower‐level PV anomalies on the overall role of the wave develo

ISSN:0035-9009    

DOI:10.1002/qj.49712656908

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractA detailed case study of mountain waves observed over the South Island of New Zealand during the Southern Alps Experiment is presented. The purpose of the study is to document the waves generated over this region, to explore the extent to which high‐resolution radiosonde data can be used to determine gravity‐wave parameters, and to estimate the vertical flux of horizontal momentum produced by flow over the Southern Alps.Organized wave clouds were observed in a visible satellite image on 20 October 1996 and are thought to have been produced by partially trapped lee waves. Using background flow fields derived from a radiosonde sounding, the theoretical horizontal wavelength of the partially trapped mode is calculated and agrees well with that measured from the satellite image.A technique, called thehorizontal projection method, is developed to analyse radiosonde soundings for mountain waves. Specifically, the technique determines the dominant horizontal wavelength over the entire depth of a radiosonde sounding. This method uses a coordinate transformation to take account of vertical changes in the background flow and, hence, vertical variations in the vertical wave number, as well as changes in the path of the radiosonde. This method is applied to a radiosonde sounding to identify the dominant wave mode, and emphasizes that radiosonde soundings should not, in general, be treated as instantaneous vertical profiles when analysing mountain waves.The mountain waves observed are simulated using a high‐resolution numerical model and the results are used to check the consistency of the analysis. The numerical model produces a horizontally averaged value of momentum flux in the lower stratosphere of −0.02 N m−2although this value is probably underestimated by at least a facto

ISSN:0035-9009    

DOI:10.1002/qj.49712656909

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractA study of the dynamical formation mechanisms of atmospheric boundary‐layer vortices is being conducted. These vortices are typically manifest as dust devils although there is evidence that they exist with some frequency in the absence of visible flow tracers. For example, in 1997 MacPherson and Belts pointed out instrument observations of invisible boundary‐layer vertical vortices over the boreal forest. Several possible mechanisms for vortex formation under a variety of different dynamical regimes are described. Most observational investigators have reported that dust devils form in environments characterized by low wind speeds. The most intriguing unknown is the source of vorticity for the formation of vortices for convection in the absence (or near absence) of mean winds. Vertical vortex formation in convection without a mean shear has not often been documented in laboratory or numerical simulations.A Large‐Eddy Simulation of the convective boundary layer is performed, without mean winds, with the purpose of examining vertical vortex‐format ion mechanisms. The current work emphasizes the conjectured larger (convective) scale vorticity‐generating mechanisms. The results indicate that vertical vortices form at some, but not all of the vertices of the simulated pattern of open convective cells. The columnar vortices may be essentially vertical or tilt with height, and may or may not extend to the surface. It is suggested that the vertical vorticity initially available to the vortices comes from the tilting of horizontal vorticity associated with local unidirectional shears due to the convective‐cell circulations, with convective‐cell asymmetries being important to the process. Subsequent evolution apparently results in the co‐location of vertical vorticity centres with local u

ISSN:0035-9009    

DOI:10.1002/qj.49712656910

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractA tethered kite balloon with multiple instrumented probes clamped to the tethering cable is used to investigate atmospheric density‐current fronts passing through three sites at varying distances from the coast. The system allows high‐frequency measurements of the three components of wind, temperature and humidity to be made simultaneously at up to ten levels as the front passes through. This enables a two‐dimensional visualization of the current and its circulation to be constructed. As a result the structure of some of the wide variety of atmospheric density currents is revealed by direct observation. The internal circulations and the effects of varying environments on the currents and their movement are described, together with turbulence statistics. One current was associated with what may have been a coastal trapped disturbance, and in one case two opposing sea breezes appear to have collided inland. Two currents penetrating into surface inversions were dissociated from the surface. One of these was travelling downwind and formed a strong near‐surface jet. Associated wave and bore structures were found to be travelling in layers above the currents in two cases. In one case environmental air was found to intrude into the head region and convect internally, while, in another, air from the head region extruded into the environmental flow. The measurement array enabled the different terms of the dynamic equations to be evaluated and their relative contribution assessed. A remarkable correlation is found between thermal and dynamic variables. A study of the internal flux balance of daytime density currents shows that the internal flow plays a significant part in maintaining the capping inversion of the current. Eddies were observed at interfacial regions in which Richardson numbers were significantly greater than 0.25, confirming some modelling studies. Frontal slopes in most cases are in accord with theoretical values but in one downwind case the slope is considerably less than predicted. The inland currents are found to be deeper than coastal currents which may explain the increase in speed observed later in the day. Water vapour frontal structures are generally less marked than thermal

ISSN:0035-9009    

DOI:10.1002/qj.49712656911

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY